Signaling system



March 3, 1936. R. L. DAVIS 2,032,921

SIGNALING SYSTEM Original Filed May 26, 1928 INVENTOR Robe/f L. 0a V1.6.

WITNESSES:

' ATTOR EY \7L I I Patented Mar. 3, 1936 UNITED STATES PATENT OFFICE SIGNALING SYSTEM Robert L. Davis, Grand Rapids, Mich., assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania My invention relates to radio apparatus and particularly to modulation circuits and coupling circuits for coupling a single tube amplifier to a push-pull amplifier.

This application is a division of my application Serial No. 280,696, filed May 26, 1928, patented Mar. 15, 1932, No. 1,849,792, and assigned to the Westinghouse Electric and Manufacturing Company.

At the present time, substantially all radioprogram transmission is accomplished by causing the amplitude of a radiated carrier-wave to vary in accordance with the speech or music being transmitted. The preferred means for effecting the amplitude variation, or modulation, of the carrier wave is generally credited to Heising, and comprises a thermionic oscillation generator and a thermionic modulator, the anodes of which are supplied with potential from a common source. An audio-frequency choke coil isincluded in the common lead to the anodes of the oscillator and modulator devices, in order that the current taken thereby shall remain constant. When signal frequencies are impressed on the input circuit of the modulator device, therefore, the potential applied to the anode of the oscillator varies in accordance therewith, thus causing the amplitude of the generated and radiated oscillations to vary proportionally to the said signal frequencies.

It is substantially impossible to secure complete modulation with the Heising constant-current system, however, since the entire cessation of oscillations in the oscillator device presupposes zero potential on the plate thereof and also zero potential on the plate of the modulator device during the intervals the latter is drawing maximum plate current. It has, accordingly, been proposed to operate the oscillator tubes at a lower plate potential than the modulator tubes in such manner that, when the oscillator plate potential falls to zero, for 100% modulation, there is still sufficient residual plate potential on the modulator tubes to permit them to draw current.

Several systems have been devised for accomplishing modulation in the manner just referred to but all of them with which I am familiar have the disadvantage of requiring separate and distinct high-potential supplies for the oscillator and modulator tubes.

In designing a transmitter employing my improved modulation system, I have found it desirable to utilize an improved formofv coupling 6 Claims. (01. 179-171) between a single tube amplifier and a push-pull amplifier in the audio-frequency amplifier.

An object of my invention is to provide a signaling system comprising means for accomplishing the complete modulation of the output thereof in accordance with signals at audio, or other, frequencies.

Another object of my invention is to provide an inter-stage coupling network particularly adapted for use in connection with signaling systems wherein audio-frequency currents, at large amplitudes, are handled.

Still another and more specific object of my invention is to provide an improved circuit for coupling a single tube amplifier to a push-pull amplifier.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with further objects and advantages thereof, will best be understood by reference to the following description of a specific embodiment, taken with the accompanying drawing, in which the single figure is a diagrammatic view of a complete radio signaling system comprising one embodiment of my invention.

The signaling system illustrated in the drawing may, for purposes of convenience in de- 30 scription, be divided into a modulator-oscillator portion which handles oscillatory currents at high frequencies and high potentials and a speech-amplifier portion which handles only currents at audio or other low frequencies. 35

The amplifier portion comprises a microphone I or an analogous device, which is connected in series with the primary winding 2 of an input transformer, the secondary winding 3 of which is connected between the grid and filament of a low-power amplifier tube 4. Plate potential for the amplifier tube is supplied from a source 5 over a circuit including an audio-frequency choke coil 6.

A plurality of amplifier tubes l0 and II, of slightly higher power than the first amplifier tube 4, are connected in push-pull or balanced relation and are supplied with filament potential from a source l2 which source also supplies filament potential for the first amplifier tube. The 50 input circuit for the tubes In and II comprises an auto-transformer I 3, an intermediate point on which is connected through a source T4 of grid-biasing potential to the cathodes" of the said tubes. The windings of the auto-transformer are .(not shown).

preferably inter-leaved to reduce the leakage reactance thereof.

One terminal of the auto-transformer is connected, through a large condenser, to the plate of the first amplifier tube, whereby the potentials at audio-frequency which are developed across the choke coil 6 are impressed across that winding of the auto-transformer included between the grid of the amplifier tube It! and the filament thereof.

The output circuit for the amplifier tubes l0 and H comprises the primary winding l5 of an output transformer, an intermediate point on the primary winding being connected to the potential source 5 in the customary manner.

I have thus provided a very convenient means whereby a stage of ordinary single-tube amplification may be coupled to a push-pull power amplifier without the necessity of using an expensive double-winding transformer. In addition, inasmuch as the leakage reactance of the autotransformer may be made extremely low, an amplifier constructed according to my invention has much better characteristics than when the usual three-winding push-pull transformer is used as a coupling device.

The modulator-oscillator portion of the signaling system preferably comprises a high-power water-cooled thermionic modulator tube 20 which is supplied with filament potential from a small generator 2|. The secondary winding 22 of the push-pull output transformer is included between the grid of the modulator tube and an intermediate point on a balancing resistor 23 connected in shunt to the filament generator 2 i. A source 24 of biasing potential is preferably included in the input circuit of the modulator tube in order that the grid thereof may be maintained at the best operating point.

An oscillator tube 30 having a grid 3|, a plate 32, and a filament 33, is supplied with filament potential from a small generator 34. A resistor 35 is connected in shunt to the terminals of the generator 34, and an intermediate point thereon is connected to the intermediate point on the resistor 23 shunting the modulator-filament-potential generator over a circuit including a plurality of small generators 36 and 31, the armatures of which are mechanically inter-connected and arranged to be driven by a constant-speed motor 38 on a main motor-generator set.

The positive terminal ofa common source 40 of high potential is connected to the plates of the modulator and oscillator tubes through an audiofrequencychoke coil 4|, while the negative terminal of thesource is connected to an intermediate point on a conductor 42 which connects the generators 36 and 31. The intermediate point is grounded to maintain it at zero potential.

The oscillator tube 30 is provided with an oscillatory output circuit comprising an inductor 43 having a tuning condenser connected in shunt thereto. Separated points on the inductor 43 are connected to an inductor 45 comprised in a radiating structure or to later amplifier stages The grid 3| and the plate 32 of the oscillator tube are connected to opposite terminals of the output inductor 43 through a plurality of blocking condensers 46 and 41, and an intermediate point on the output inductor is connected, by a conductor 48, to the resistor shunting the oscillator tube filament-potential generator.

In the operation of the system illustrated, the balancing set, comprising the generators 36 and 31, maintains the filaments of the modulator and oscillator tubes below and above ground potential, respectively. The space current in the oscillator tube flows against the potential developed by the generator 36, causing the latter to act as a motor to supply power to the generator 31, in addition to the power supplied thereto from the driving motor. The generators 36 and 31, therefore, serve as energy-converting devices, functioning to maintain a greater potential difference between the plate and filament of the modulator tube than that between the plate and filament of the oscillator tube.

By proper adjustment of the field control (not shown) of the balancing-set machines and the oscillator and modulator plate currents, the driving motor 38 could be dispensed with. However, in order to provide for greater latitude in adjustment and improved stability, it is desirable to make use of the driving motor, as shown. In some instances, it is possible that the driving motor might be required to absorb energy and to return it to the line. I

7 Although I have illustrated and described only a single embodiment of my invention, many modi fications will be apparent to persons skilled in the art. My invention, therefore, is not to be limited except insofar as isnecessitated by the priorart and by the spirit of the appended claims.

I claim as my invention:

1. An amplifier system comprising a singletube amplifier having an input circuit and an output circuit, an amplifier comprising two tubes each having a control-electrode and a cathode and having a push-pull input circuit and an output circuit, and means comprising an auto-transformer for coupling the output circuit of said single-tube amplifier to said push-pull input circuit.

2. An amplifier system comprising. a singletube amplifier, a push-pull amplifier, and means for coupling the output of said single-tube amplie fier to the input of said push-pull amplifier, said means comprising an auto-transformer.

3. An electrical amplification system comprising in combination, an electron discharge device adapted to have its grid and cathode elements connected to an input circuit, an anode choke coil, said discharge device having its anode supplied with positive potential through said anode choke coil, a condenser, and transformer means, said transformer means having a primary winding and a secondary winding, said condenser and said primary winding being arranged inparallel thermionic tubes having cathode-grid circuits, transformer means having primary and secondary windings, said transformer means having its primary winding included in one of said circuits,- and its'secondary winding included in the other of said circuits, a coupling condenser for coupling said thermionic tubes to a signal circuit, a

plate choke coil, said plate choke coil being connected to said coupling condenser, and said transformer meansprimary winding serving asa choke coil for one of said cathode-grid circuits.

5. In an audio amplifier circuit, in combina- 75 tion, a pair of electron discharge devices, each of said devices having a grid element together with a cathode and anode, external circuits extending between the cathode and grid elements of each of said devices, and transformer means having a primary winding and a secondary winding solidly connected in series, said transformer means having its primary winding connected in the external circuit of one of said devices and its secondary winding connected in the external circuit of the other of said devices, and a coupling circuit connected at one side to the primary winding of said transformer means and at the other side to the cathode elements of said devices, whereby the primary Winding of said transformer means is arranged in series with said coupling circuit.

6. In an audio amplifier circuit, in combination, a pair of electron discharge devices connected in opposition, each of said devices having a grid element together with a cathode and anode, external circuits extending between the cathode and grid elements of each of said devices, and transformer means having a primary winding and. a secondary winding solidly connected in series, said transformer means having its primary winding connected in the external circuit of one of said devices and its secondary winding connected in the external circuit of the other of said devices, and a coupling circuit having a condenser therein, said coupling circuit being connected at one side to the primary Winding of said transformer means and at the other side to the cathode elements of said devices, whereby said condenser and the primary winding of said transformer means are arranged in series in said coupling circuit.

ROBERT L. DAVIS. 

